Gas turbines consume large quantities of air. Air contains particles in the form of aerosols which are drawn into the compressor of the gas turbine with the air flow. A majority of these particles accompany the air flow and leave the gas turbine with the exhaust gases. However, some particles tend to adhere to components in the channels of the gas turbine. These particles form a deposit on the components, thus deteriorating the aerodynamic properties. As with increased roughness of the surface, the coating causes a change in the boundary layer flow along the surface. The coating, i.e. the increased roughness of the surface, results in pressure step-up losses and a reduction in the amount of air the compressor compresses. For the compressor as a whole this entails deteriorated efficiency, reduced mass flow and reduced final pressure. Modern gas turbines are equipped with filters to filter the air in front of the entrance to the compressor. These filters can catch only some of the particles. To maintain economic operation of the gas turbine, therefore, it has been found necessary to regularly clean the surface of the compressor components in order to maintain good aerodynamic properties.
Various methods for cleaning gas turbine compressors are already known. Injecting crushed nut shells into the air flow to the compressor has been found practically feasible. The drawback is that the nut-shell material may find its way into the internal air system of the gas turbine and result in clogging of ducts and valves.
Another cleaning method is based on wetting the compressor components with a washing fluid by spraying drops of the washing fluid into the air intake to the compressor. The washing fluid may consist of water or water mixed with chemicals. In the known cleaning method the gas turbine rotor is rotated with the aid of the start motor of the gas turbine. This method is known as “crank washing” or “off-line washing” and is characterised in that the gas turbine does not burn fuel during cleaning. The spray is produced by the cleaning fluid being pumped through nozzles which atomize the fluid. The nozzles are installed on the walls of the air duct upstream of the compressor inlet, or are installed on a frame placed temporarily in the intake duct.
The method results in the compressor components being drenched in cleaning fluid and the dirt particles being detached by the chemical effects of the chemicals, as well as mechanical forces deriving from rotation of the rotor. The method is considered both efficient and useful. The rotor speed during crank washing is a fraction of that at normal operation of the gas turbine. An important feature with crank washing is that the rotor rotates at low speed so that there is little risk of mechanical damage.
A method known from U.S. Pat. No. 5,011,540 is based on the compressor components being wetted with cleaning fluid while the gas turbine is in operation, i.e. while fuel is being burned in the combustion chamber of the gas turbine unit. The method is known as “on-line washing” and, in common, with crank washing, a washing fluid is injected upstream of the compressor. This method is not as efficient as crank washing. The lower efficiency is a result of poorer cleaning mechanisms prevailing at higher rotor speeds and high air speeds when the gas turbine is in operation. A specific quantity of washing fluid should be injected since too much washing fluid may cause mechanical damage in the compressor and too little washing fluid results in poor soaking of the compressor components. Another problem with the on-line washing method is that the washing fluid must not only be caught by the blade surface and guide vanes of the first step, it must also be distributed to the compressor step downstream of the first step. If a large proportion of the washing fluid is caught by the blade surface of the first step, the washing fluid will be moved to the periphery of the rotor due to centrifugal forces and will therefore no longer participate in the cleaning process.
The object of the invention is to fully or partially eliminate said problems.